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I'm using an object's __del__() to unsubscribe it from an event (using an event scheme similar to this):
import my_enviroment
class MyClass():
def __del__(self):
my_environment.events.my_event -= self.event_handler_func
Oddly I received the following error at the end of the program's run:
Exception AttributeError: "'NoneType' object has no attribute 'events'" in <bound method MyClass.__del__ of <myclass.MyClass instance at 0x04C54580>> ignored
How could this be possible?! my_environment is a module I imported, how come it could be None? (events is a global object in it with event hooks such as my_event)
306
The purpose of __all__ is simply to define which symbols should be imported as part of a wildcard import that targets that module. If a symbol is importable through a wildcard import, it makes sense that it should be considered public.
In order to import a module, the directory having that module must be present on PYTHONPATH. It is an environment variable that contains the list of packages that will be loaded by Python. The list of packages presents in PYTHONPATH is also present in sys. path, so will add the parent directory path to the sys.
When we import a module the Python interpreter searches for the module in the following manner: First, it searches for the module in the current directory. If the module isn't found in the current directory, Python then searches each directory in the shell variable PYTHONPATH.
According to the python doc about __del__ :
[...] other globals referenced by the
__del__()method may already have been deleted or in the process of being torn down (e.g. the import machinery shutting down). For this reason,__del__()methods should do the absolute minimum needed to maintain external invariants.
In other words, when the __del__ method is called on your object, the my_enviroment may have been 'deleted' by python, so it can be None...
127
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